Posted on by Nancy Atkinson

The Earth and Moon May Have Formed Later Than Previously Thought

The collision between "Proto-Earth" and Theia, from which the Earth and Moon were created 4,500-4,400 million years ago. Both planets had a massive iron core when they collided and created the Moon and Earth.

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The Earth and Moon were created as the result of a giant collision between two planets the size of Mars and Venus. Until now it was thought to have happened when the solar system was 30 million years old or approximately 4.5 billion years ago. But new research shows that the Earth and Moon may have formed much later – perhaps up to 150 million years after the formation of the solar system.

“We have determined the ages of the Earth and the Moon using tungsten isotopes, which can reveal whether the iron cores and their stone surfaces have been mixed together during the collision,” said Tais W. Dahl, from the Niels Bohr Institute at the University of Copenhagen in collaboration with professor David J. Stevenson from the California Institute of Technology (Caltech).

The planets in the solar system were created by collisions between planetary embryos orbiting the newborn sun. In the collisions the small planets congealed together and formed larger and larger planets. When the gigantic collision occurred that ultimately formed the Earth and Moon, it happened at a time when both planetary bodies had a core of metal (iron) and a surrounding mantle of silicates (rock). But when did it happen and how did it happen? The collision took place in less than 24 hours and the temperature of the Earth was so high (7000º C), that both rock and metal must have melted in the turbulent collision. But were the stone mass and iron mass also mixed together?

The age of the Earth and Moon can be dated by examining the presence of certain elements in the Earth’s mantle. Hafnium-182 is a radioactive substance, which decays and is converted into the isotope tungsten-182. The two elements have markedly different chemical properties and while the tungsten isotopes prefer to bond with metal, hafnium prefers to bond to silicates, i.e. rock.

It takes 50-60 million years for all hafnium to decay and be converted into tungsten, and during the Moon forming collision nearly all the metal sank into the Earth’s core. But did all the tungsten go into the core?

“We have studied to what degree metal and rock mix together during the planet forming collisions. Using dynamic model calculations of the turbulent mixing of the liquid rock and iron masses we have found that tungsten isotopes from the Earth’s early formation remain in the rocky mantle,” said Tahl.

The new studies imply that the moon forming collision occurred after all of the hafnium had decayed completely into tungsten.

“Our results show that metal core and rock are unable to emulsify in these collisions between planets that are greater than 10 kilometers in diameter and therefore that most of the Earth’s iron core (80-99 %) did not remove tungsten from the rocky material in the mantle during formation” said Dahl.

The result of the research means that collision that created the Earth and the Moon may have occurred as much as 150 million years after the formation of the solar system, much later than the 30 million years that was previously thought.

The research results have been published in the scientific journal, Earth and Planetary Science Letters.

From a University of Copenhagen press release.

Posted on by Steve Nerlich

Astronomy Without A Telescope – The Nice Way To Build A Solar System

When considering how the solar system formed, there are a number of problems with the idea of planets just blobbing together out of a rotating accretion disk. The Nice model (and OK, it’s pronounced ‘niece’ – as in the French city) offers a better solution.

In the traditional Kant/Laplace solar nebula model you have a rotating protoplanetary disk within which loosely associated objects build up into planetesimals, which then become gravitationally powerful centres of mass capable of clearing their orbit and voila planet!

It’s generally agreed now that this just can’t work since a growing planetesimal, in the process of constantly interacting with protoplanetary disk material, will have its orbit progressively decayed so that it will spiral inwards, potentially crashing into the Sun unless it can clear an orbit before it has lost too much angular momentum.

The Nice solution is to accept that most planets probably did form in different regions to where they orbit now. It’s likely that the current rocky planets of our solar system formed somewhat further out and have moved inwards due to interactions with protoplanetary disk material in the very early stages of the solar system’s formation.

It is likely that within 100 million years of the Sun’s ignition, a large number of rocky protoplanets, in eccentric and chaotic orbits, engaged in collisions – followed by the inward migration of the last four planets left standing as they lost angular momentum to the persisting gas and dust of the inner disk. This last phase may have stabilised them into the almost circular, and only marginally eccentric, orbits we see today.

The hypothesized collision between 'Earth Mk 1' and Theia may have occurred late in rocky planet formation creating the Earth as we know it with its huge Moon of accreted impact debris

Meanwhile, the gas giants were forming out beyond the ‘frost line’ where it was cool enough for ices to form. Since water, methane and CO2 were a lot more abundant than iron, nickel or silicon – icy planetary cores grew fast and grew big, reaching a scale where their gravity was powerful enough to hold onto the hydrogen and helium that was also present in abundance in the protoplanetary disk. This allowed these planets to grow to an enormous size.

Jupiter probably began forming within only 3 million years of solar ignition, rapidly clearing its orbit, which stopped it from migrating further inward. Saturn’s ice core grabbed whatever gases Jupiter didn’t – and Uranus and Neptune soaked up the dregs. Uranus and Neptune are thought to have formed much closer to the Sun than they are now – and in reverse order, with Neptune closer in than Uranus.

And then, around 500 million years after solar ignition, something remarkable happened. Jupiter and Saturn settled into a 2:1 orbital resonance – meaning that they lined up at the same points twice for every orbit of Saturn. This created a gravitational pulse that kicked Neptune out past Uranus, so that it ploughed in to what was then a closer and denser Kuiper Belt.

The result was a chaotic flurry of Kuiper Belt Objects, many being either flung outwards towards the Oort cloud or flung inwards towards the inner solar system. These, along with a rain of asteroids from a gravitationally disrupted asteroid belt, delivered the Late Heavy Bombardment which pummelled the inner solar system for several hundred million years – the devastation of which is still apparent on the surfaces of the Moon and Mercury today.

Then, as the dust finally settled around 3.8 billion years ago and as a new day dawned on the third rock from the Sun – voila life!

Posted on by Steve Nerlich

Astronomy Without A Telescope – One Potato, Two Potato

Sometimes it’s good to take a break from mind-stretching cosmology models, quantum entanglements or events at 10-23 seconds after the big bang and get back to some astronomy basics. For example, the vexing issue of the potato radius. 

At the recent 2010 Australian Space Science Conference, it was proposed by Lineweaver and Norman that all naturally occurring objects in the universe adopt one of five basic shapes depending on their size, mass and dynamics. Small and low mass objects can be considered Dust – being irregular shapes governed primarily by electromagnetic forces. 

Next up are Potatoes, being objects where accretion by gravity begins to have some effect, though not as much as in the more massive Spheres – which, to quote the International Astronomical Union’s second law of planets, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. 

Objects of the scale of molecular dust clouds will collapse down into Disks where the sheer volume of accreting material means that much of it can only rotate in a holding pattern around and towards the centre of mass. Such objects may evolve into a star with orbiting planets (or not), but the initial disk structure seems to be a mandatory step in the formation of objects at this scale. 

At the galactic scale you may still have disk structures, such as a spiral galaxy, but usually such large scale structures are too diffuse to form accretion disks and instead cluster in Halos – of which the central bulge of a spiral galaxy is one example. Other examples are globular clusters, elliptical galaxies and even galactic clusters. 

The proposed five major forms that accumulated matter adopts in our universe. Credit: Lineweaver and Norman.

The authors then investigated the potato radius, or Rpot, to identify the transition point from Potato to Sphere, which would also represent the transition point from small celestial object to dwarf planet. Two key issues emerged in their analysis. 

Firstly, it is not necessary to assume a surface gravity of a magnitude necessary to generate hydrostatic equilibrium. For example, on Earth such rock crushing forces only act at 10 kilometres or more below the surface – or to look at it another way you can have a mountain on Earth the size of Everest (9 kilometres), but anything higher will begin to collapse back towards the planet’s roughly spheroid shape. So, there is an acceptable margin where a sphere can still be considered a sphere even if it does not demonstrate complete hydrostatic equilibrium across its entire structure. 

Secondly, the differential strength of molecular bonds affects the yield strength of a particular material (i.e. its resistance to gravitational collapse). 

On this basis, the authors conclude that Rpot for rocky objects is 300 kilometres. However, Rpot for icy objects is only 200 kilometres, due to their weaker yield strength, meaning they more easily conform to a spheroidal shape with less self-gravity. 

Since Ceres is the only asteroid with a radius that is greater than Rpot for rocky objects we should not expect any more dwarf planets to be identified in the asteroid belt. But applying the 200 kilometre Rpot for icy bodies, means there may be a whole bunch of trans-Neptunian objects out there that are ready to take on the title.

Posted on by Nancy Atkinson

How Common are Solar Systems Like Ours?

Solar system montage. Credit: NASA

On the whole, we’d like to think we’re special, but we also hope we aren’t alone in the Universe. Astronomers have been trying to figure out just how common solar systems like ours are across the cosmos, and during one moment of epiphany one scientist figured out how to make the calculations. It took a worldwide collaboration of astronomers to do the work, but they concluded that about 10 – 15 percent of stars in the universe host systems of planets like our own, with several gas giant planets in the outer part of the solar system.

“Now we know our place in the universe,” said Ohio State University astronomer Scott Gaudi. “Solar systems like our own are not rare, but we’re not in the majority, either.”

The find comes from a collaboration headquartered at Ohio State called the Microlensing Follow-Up Network (MicroFUN), which searches the sky for extrasolar planets.

MicroFUN astronomers use gravitational microlensing — which occurs when one star happens to cross in front of another as seen from Earth. The nearer star magnifies the light from the more distant star like a lens. If planets are orbiting the lens star, they boost the magnification briefly as they pass by.

During his talk at the American Astronomical Society meeting in Washington, DC today, Gaudi said, “Planetary microlensing basically is looking for planets you can’t see around stars you can’t see.”

This method is especially good at detecting giant planets in the outer reaches of solar systems — planets analogous to our own Jupiter.

This latest MicroFUN result is the culmination of 10 years’ work — and one sudden epiphany, explained Gaudi and Andrew Gould, professor of astronomy at Ohio State.

Ten years ago, Gaudi wrote his doctoral thesis on a method for calculating the likelihood that extrasolar planets exist. At the time, he concluded that less than 45 percent of stars could harbor a configuration similar to our own solar system.

Then, in December of 2009, Gould was examining a newly discovered planet with Cheongho Han of the Institute for Astrophysics at Chungbuk National University in Korea. The two were reviewing the range of properties among extrasolar planets discovered so far, when Gould saw a pattern.

“Basically, I realized that the answer was in Scott’s thesis from 10 years ago,” Gould said. “Using the last four years of MicroFUN data, we could add a few robust assumptions to his calculations, and we could now say how common planet systems are in the universe.”

The find boils down to a statistical analysis: in the last four years, the MicroFUN survey has discovered only one solar system like our own — a system with two gas giants resembling Jupiter and Saturn, which astronomers discovered in 2006 and reported in the journal Science in 2008.

“We’ve only found this one system, and we should have found about eight by now — if every star had a solar system like Earth’s,” Gaudi said.

The slow rate of discovery makes sense if only a small number of systems — around 10 percent — are like ours, they determined.

“While it is true that this initial determination is based on just one solar system and our final number could change a lot, this study shows that we can begin to make this measurement with the experiments we are doing today,” Gaudi added.

As to the possibility of life as we know it existing elsewhere in the universe, scientists will now be able to make a rough guess based on how many solar systems are like our own.

Our solar system may be a minority, but Gould said that the outcome of the study is actually positive.

“With billions of stars out there, even narrowing the odds to 10 percent leaves a few hundred million systems that might be like ours,” he said.

At the AAS conference today, Gaudi was awarded the Helen B. Warner Prize for Astronomy.

Source: AAS, EurekAlert

Posted on by Fraser Cain

Planets Fact Sheet

Mercury
Mass: 0.3302 x 1024 kg
Volume: 6.083 x 1010 km3
Average radius: 2439.7 km
Average diameter: 4879.4 km
Mean density: 5.427 g/cm3
Escape velocity: 4.3 km/s
Surface gravity: 3.7 m/s2
Visual magnitude: -0.42
Natural satellites: 0
Rings? – No
Semimajor axis: 57,910,000 km
Orbit period: 87.969 days
Perihelion: 46,000,000 km
Aphelion: 69,820,000 km
Mean orbital velocity: 47.87 km/s
Maximum orbital velocity: 58.98 km/s
Minimum orbital velocity: 38.86 km/s
Orbit inclination: 7.00°
Orbit eccentricity: 0.2056
Sidereal rotation period: 1407.6 hours
Length of day: 4222.6 hours
Discovery: Known since prehistoric times
Minimum distance from Earth: 77,300,000 km
Maximum distance from Earth: 221,900,000 km
Maximum apparent diameter from Earth: 13 arc seconds
Minimum apparent diameter from Earth: 4.5 arc seconds
Maximum visual magnitude: -1.9

Venus
Mass: 4.8685 x 1024 kg
Volume: 92.843 x 1010 km3
Average radius: 6051.8 km
Average diameter: 12103.6 km
Mean density: 5.243 g/cm3
Escape velocity: 10.36 km/s
Surface gravity: 8.87 m/s2
Visual magnitude: -4.40
Natural satellites: 0
Rings? – No
Semimajor axis: 108,210,000 km
Orbit period: 224.701 days
Perihelion: 107,480,000 km
Aphelion: 108,940,000 km
Mean orbital velocity: 35.02 km/s
Maximum orbital velocity: 35.26 km/s
Minimum orbital velocity: 34.79 km/s
Orbit inclination: 3.39°
Orbit eccentricity: 0.0067
Sidereal rotation period: 5832.5 hours
Length of day: 2802.0 hours
Discovery: Known since prehistoric times
Minimum distance from Earth: 38,200,000 km
Maximum distance from Earth: 261,000,000 km
Maximum apparent diameter from Earth: 66.0 arc seconds
Minimum apparent diameter from Earth: 9.7 arc seconds
Maximum visual magnitude: -4.6

Earth
Mass: 5.9736 x 1024 kg
Volume: 108.321 x 1010 km3
Average radius: 6,371.0 km
Average diameter: 12,742 km
Mean density: 5.515 g/cm3
Escape velocity: 11.186 km/s
Surface gravity: 9.798 m/s2
Visual magnitude: -3.86
Natural satellites: 1
Rings? – No
Semimajor axis: 149,600,000 km
Orbit period: 365.256 days
Perihelion: 147,090,000 km
Aphelion: 152,100,000 km
Mean orbital velocity: 29.78 km/s
Maximum orbital velocity: 30.29 km/s
Minimum orbital velocity: 29.29 km/s
Orbit inclination: 0.00°
Orbit eccentricity: 0.0167
Sidereal rotation period: 23.9345 hours
Length of day: 24.0000 hours
Axial tilt: 23.45°

Mars
Mass: 0.64185 x 1024 kg
Volume: 16.318 x 1010 km3
Average radius: 3,389.5 km
Average diameter: 6,779 km
Mean density: 3.933 g/cm3
Escape velocity: 5.03 km/s
Surface gravity: 3.71 m/s2
Visual magnitude: -1.52
Natural satellites: 2
Rings? – No
Semimajor axis: 227,920,000 km
Orbit period: 686.980 days
Perihelion: 206,620,000 km
Aphelion: 249,230,000 km
Mean orbital velocity: 24.13 km/s
Orbit inclination: 1.850°
Orbit eccentricity: 0.0935
Sidereal rotation period: 24.6229 hours
Length of day: 24.6597 hours
Axial tilt: 25.19 °
Discovery: Known since prehistoric times
Minimum distance from Earth: 55,700,000 km
Maximum distance from Earth: 401,300,000 km
Maximum apparent diameter from Earth: 25.1 arc seconds
Minimum apparent diameter from Earth: 3.5 arc seconds
Maximum visual magnitude: -2.91

Jupiter
Mass: 1,898.6 x 1024 kg
Volume: 143,128 x 1010 km3
Average radius: 69,911 km
Average diameter: 139,822 km
Mean density: 1.326 g/cm3
Escape velocity: 59.5 km/s
Surface gravity: 24.79 m/s2
Natural satellites: 63
Rings? – Yes
Semimajor axis: 778,570,000 km
Orbit period: 4,332.589 days
Perihelion: 740,520,000 km
Aphelion: 816,620,000 km
Mean orbital velocity: 13.07 km/s
Orbit inclination: 1.304°
Orbit eccentricity: 0.0489
Sidereal rotation period: 9.9250 hours
Length of day: 9.9259 hours
Axial tilt: 3.13°
Discovery: Known since prehistoric times
Minimum distance from Earth: 588,500,000 km
Maximum distance from Earth: 968,100,000 km
Maximum apparent diameter from Earth: 50.1 arc seconds
Minimum apparent diameter from Earth: 29.8 arc seconds
Maximum visual magnitude: -2.94

Saturn
Mass: 568.46 x 1024 kg
Volume: 82,713 x 1010 km3
Average radius: 58,232 km
Average diameter: 116,464 km
Mean density: 0.687 g/cm3
Escape velocity: 35.5 km/s
Surface gravity: 10.44 m/s2
Natural satellites: 60
Rings? – Yes
Semimajor axis: 1,433,530,000 km
Orbit period: 10,759.22 days
Perihelion: 1,352,550,000 km
Aphelion: 1,514,500,000 km
Mean orbital velocity: 9.69 km/s
Orbit inclination: 2.485°
Orbit eccentricity: 0.0565
Sidereal rotation period: 10.656 hours
Length of day: 10.656 hours
Axial tilt: 26.73°
Discovery: Known since prehistoric times
Minimum distance from Earth: 1,195,500,000 km
Maximum distance from Earth: 1,658,500,000 km
Maximum apparent diameter from Earth: 20.1 arc seconds
Minimum apparent diameter from Earth: 14.5 arc seconds
Maximum visual magnitude: 0.43

Uranus
Mass: 86.832 x 1024 kg
Volume: 6,833 x 1010 km3
Average radius: 25,362 km
Average diameter: 50,724 km
Mean density: 1.270 g/cm3
Escape velocity: 21.3 km/s
Surface gravity: 8.87 m/s2
Natural satellites: 27
Rings? – Yes
Semimajor axis: 2,872,460,000 km
Orbit period: 30,685.4 days
Perihelion: 2,741,300,000 km
Aphelion: 3,003,620,000 km
Mean orbital velocity: 6.81 km/s
Orbit inclination: 0.772°
Orbit eccentricity: 0.0457
Sidereal rotation period: 17.24 hours
Length of day: 17.24 hours
Axial tilt: 97.77°
Discovery: 13 March 1781
Minimum distance from Earth: 2,581,900,000 km
Maximum distance from Earth: 3,157,300,000 km
Maximum apparent diameter from Earth: 4.1 arc seconds
Minimum apparent diameter from Earth: 3.3 arc seconds
Maximum visual magnitude: 5.32

Neptune
Mass: 102.43 x 1024 kg
Volume: 6,254 x 1010 km3
Average radius: 24,622 km
Average diameter: 49,244 km
Mean density: 1.638 g/cm3
Escape velocity: 23.5 km/s
Surface gravity: 11.15 m/s2
Natural satellites: 13
Rings? – Yes
Semimajor axis: 4,495,060,000 km
Orbit period: 60,189 days
Perihelion: 4,444,450,000 km
Aphelion: 4,545,670,000 km
Mean orbital velocity: 5.43 km/s
Orbit inclination: 1.769°
Orbit eccentricity: 0.0113
Sidereal rotation period: 16.11 hours
Length of day: 16.11 hours
Axial tilt: 28.32°
Discovery: 23 September 1846
Minimum distance from Earth: 4,305,900,000 km
Maximum distance from Earth: 4,687,300,000 km
Maximum apparent diameter from Earth: 2.4 arc seconds
Minimum apparent diameter from Earth: 2.2 arc seconds
Maximum visual magnitude: 7.78

We’ve written many articles about the Solar System. Here’s an article about how many moons there are in the Solar System, and here’s an article about the formation of the Solar System.

If you’d like more info on the Solar System, check out NASA’s Planetary Fact Sheet.

We’ve recorded several episodes of Astronomy Cast about the Solar System. Start here, Episode 49: Mercury.

Posted on by Fraser Cain

Why is Venus So Hot?

You might have heard that Venus is the hottest planet in the Solar System. In fact, down at the surface of Venus it’s hot enough to melt lead. But why is Venus so hot?

Three words: runaway greenhouse effect. In many ways, Venus is a virtual twin of Earth. It has a similar size, mass and gravity as well as internal composition. But the one big difference is that Venus has a much thicker atmosphere. If you could stand on the surface of Venus, you would experience 93 times the atmospheric pressure we experience here on Earth; you’d have to dive down 1 km beneath the surface of the ocean to experience that kind of pressure. Furthermore, that atmosphere is made up almost entirely of carbon dioxide. As you’ve probably heard, carbon dioxide makes an excellent greenhouse gas, trapping heat from the Sun. The atmosphere of Venus allows the light from the Sun to pass through the clouds and down to the surface of the planet, which warms the rocks. But then the infrared heat from the warmed rocks is prevented from escaping by the clouds, and so the planet warmed up.

The average temperature on Venus is 735 kelvin, or 461° C. In fact, it’s that same temperature everywhere on Venus. It doesn’t matter if you’re at the pole, or at night, it’s always 735 kelvin.

It’s believed that plate tectonics on Venus stopped billions of years ago. And without plate tectonics burying carbon deep inside the planet, it was able to build up in the atmosphere. The carbon dioxide built up to the point that any oceans on Venus boiled away. And then the Sun’s solar wind carried the hydrogen atoms away from Venus, making it impossible to ever make liquid water again. The concentration of carbon dioxide just kept increasing until it was all in the atmosphere.

We’ve written many articles about Venus for Universe Today. Here’s an article about the atmosphere of Venus, and here’s an article about how to find Venus in the sky.

If you’d like more info on Venus, here’s a cool lecture about Venus and the greenhouse effect, and here’s more information on the runaway greenhouse effect on Venus.

We’ve also recorded an entire episode of Astronomy Cast just about Venus. Listen here, Episode 50: Venus.

Posted on by Jon Voisey

Reexamining a Cataclysm

Image of Earth's Moon centered on the Orientale Basin taken by Galileo Spacecraft.

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One of the legacies of the Apollo program is the rare lunar samples it returned. These samples (along with meteorites that originated from the moon and even one from Mars) can be radiometrically dated, and together they paint a picture a cataclysmic time in the history of our solar system. Over a period of time some 3.8 to 4.1 billion years ago, the moon underwent a fierce period of impacts that was the origin of most of the craters we see today. Paired with the “Nice model” (named after the French university where it was developed, not because it was pleasant in any way), which describes the migration of planets to their current orbits, it is widely held that the migration of Jupiter or one of the other gas giants migrations during this period, caused a shower of asteroids or comets to rain down upon the inner solar system in a time known as the “Late Heavy Bombardment” (LHB).

A new paper by astronomers from Harvard and the University of British Columbia disagrees with this picture. In 2005, Strom et al. published a paper in Science which analyzed the frequency of craters of various sizes on the lunar highlands, Mars, and Mercury (since these are the only rocky bodies in the inner solar system without sufficient erosion to wash away their cratering history). When comparing relatively young surfaces which had been more recently resurfaced to older ones from the Late Heavy Bombardment area, is that there were two separate, but characteristic curves. The one for the LHB era revealed a crater frequency peaking at craters near 100 km (62 miles) in diameter and dropping off rapidly to lower diameters. Meanwhile, the younger surfaces showed a nearly even amount of craters of all sizes measurable. Additionally, the LHB impacts were an order of magnitude more common than the newer ones.

The Strom et al. took this as evidence that two different populations of impactors were at work. The LHB era, they called Population I. The more recent, they called Population II. What they noticed was the current size distribution of main belt asteroids (MBAs) was “virtually identical to the Population 1 projectile size distribution”. Additionally, since the size distribution of the MBA is the same today, this indicated that the process which sent these bodies our way didn’t discriminate based on size, which would weed out that size and alter the distribution we observed today. This ruled out processes such as the Yarkovsky effect but agreed with the gravitational shove as a large body would move through the region. The inverse of this (that a process was selecting rocks to chuck our way based on size) would be indicative of Strom’s Population II objects.

However, in this paper recently uploaded to arXiv, Cuk et al. argue that the dates of many of the regions investigated by Strom et al. cannot be reliably dated and therefore, cannot be used to investigate the nature of the LHB. They suggest that only the Imbrium and Orientale basins, which have their formation dates precisely known from rocks retrieved by Apollo missions, can be used to accurately describe the cratering history during this period.

With this assumption, Cuk’s group reexamined the frequency of crater sizes for just these basins. When this was plotted for these two groups, they found that the power law they used to fit the data had “an index of -1.9 or -2 rather than -1.2 or -1.3 (like the modern asteroid belt)”. As such, they claim, “theoretical models producing the lunar cataclysm by gravitational ejection of main-belt asteroids are seriously challenged.”

Although they call into question Strom et al.’s model, they cannot propose a new one. They suggest some causes that are unlikely, such as comets (which have too low of impact probabilities). One solution they mention is that the population of the asteroid belt has evolved since the LHB which would account for the differences. Regardless, they conclude that this question is more open ended than previously expected and that more work will need to be done to understand this cataclysm.

Posted on by Fraser Cain

Planet Jupiter

Jupiter photo. Image credit: NASA/SSI

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Jupiter is the 5th planet in the Solar System, and by far the largest, containing 2.5 times the mass of the rest of the planets in the Solar System. Everything about planet Jupiter is big. It has the most moons in the Solar System, including the largest moon, Ganymede. It has the largest storm: the Great Red Spot; the most extreme gravity, and the biggest temperature extremes.

Because you can see Jupiter with the unaided eye, it’s impossible to say who actually discovered Jupiter. But it was Galileo Galilei who first turned his rudimentary telescope on Jupiter in 1610. Even with its dim optics, Galileo was able to make out the fact that Jupiter had 4 bright moons and bands across the planet. Since astronomers believed that everything orbited around the Earth, finding moons orbiting Jupiter threw the Earth-centered theory of the Universe into doubt. Even the smallest telescope will show you what Galileo saw.

Planet Jupiter orbits the Sun at an average distance of 779 million km (484 million miles), and it takes 4,333 Earth days to complete one orbit around the Sun; that’s almost 12 years. But Jupiter rotates once on its axis every 9 hours and 56 minutes. This high rotation speed has flattened out the planet, so that its equator is much further from the center of Jupiter than the poles. Jupiter’s also the largest plane in the Solar System, with an equatorial diameter of 142,984 kilometers (88,846 miles) – 11 times the diameter of Earth.

Jupiter has 318 times more mass than Earth, but it has a relatively low density; only 1/4 the density of Earth. It has such a low density because Jupiter is made up almost entirely of hydrogen with a little bit of helium. The upper atmosphere has tiny amounts of ammonia and other chemicals, which create the bands and clouds we see in photographs. The most familiar feature in Jupiter’s atmosphere is the planet’s Great Red Spot. This is a long-lived storm large enough to swallow up three planets the size of Earth.

It also has the largest number of moons in the Solar System: 63 at last count. The 4 largest moons are the Galilean moons, named after Galileo who discovered them. Ganymede measures 3,273 km across, and it’s the largest moon in the Solar System. Io orbits the closest of these moons and its undergoing almost constant volcanic activity because of tidal flexing being so close to Jupiter. Europa and Callisto probably have oceans of liquid water underneath thick shells of ice, and could be the home to exotic forms of life. Jupiter also has its own set of rings.

Seven spacecraft from Earth have made the journey to Jupiter: Pioneer 10, Pioneer-Saturn, Voyager 1, Voyager 2, Ulysses, Galileo, and New Horizons. Pioneer 10 was the first to reach the planet, making its flyby in 1972. The Galileo spacecraft actually went into orbit around Jupiter, to study the planet and its moons in great detail.

We’ve written many articles about Jupiter for Universe Today. Here’s an article about how Jupiter might protect us in the Solar System, and here’s an article about how you can see Jupiter in a telescope.

If you’d like more info on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded an episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Reference:
NASA

Posted on by Fraser Cain

How Big is Jupiter?

Hubble Jupiter

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I’m sure you’ve heard that Jupiter is the largest planet in the Solar System, but just how big is Jupiter?

In terms of size, Jupiter is 142,984 km (88,846 miles) in diameter across its equator. If you just compare that to Earth, it’s 11.2 times the diameter of Earth. But then, it’s only 10% the diameter of the Sun.

The volume of Jupiter is 1.43128×1015 km3. That’s enough to fit inside 1321 planets the size of Earth, and still have room left over.

The surface area of Jupiter is 6.21796×1010 km2. And just for comparison, that’s 122 times more surface area than Earth.

And finally, the mass of Jupiter is 1.8986×1027 kg. That’s enough mass for 318 Earths. In fact, Jupiter is 2.5 times more than the mass of all the other planets in the Solar System. But then, the Sun accounts for 99.9% of the mass of the Solar System.

Jupiter’s big, no question, but don’t worry about the possibility that Jupiter might become a star. It would need another 80 times its own mass to be able to ignite solar fusion.

We’ve written several articles about Jupiter for Universe Today. Here’s an article about an impact that recently happened on Jupiter, and here’s an article about how Jupiter might protect us in the Solar System.

If you’d like more information on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded a whole episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Posted on by Fraser Cain

Tenth Planet: The Next World in the Solar System

Tenth planet? Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)
Tenth planet? Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)

Before 1930, there were 8 planets in the Solar System. And then with the discovery of Pluto in 1930, the total number of planets rose to 9. Although astronomers kept searching for more planets, it wasn’t until 2005 that an object larger than Pluto was found orbiting in the distant Solar System. This new object was known as Eris, and many considered it to be a tenth planet; but it actually created a controversy that ended up with Pluto being kicked out of the planet club and becoming a dwarf planet. There really is no 10th planet, in fact, we don’t even have a ninth planet any more.

Discovery of Eris

Eris, originally named 2003 ub 313 was discovered by Palomar observatory researcher Mike Brown; Mike has been behind many of the trans-Neptunian discoveries in the last decade. Mike and his team discovered Eris by systematically scanning the sky for objects moving at the right speed in the right object to be in the outer Solar System.

Further observations of Eris showed that it was actually larger than Pluto by a significant amount; it measured 2,500 km across, compared to Pluto’s 2,300 km diameter. And it orbited at a distance of 67 astronomical units, compared to Pluto’s 39 AU (1 AU is the average distance from the Earth to the Sun).

Tenth Planet, Dwarf Planet

Because there was now a larger object than Pluto found orbiting the Sun, astronomers needed to decide whether this would be come the tenth planet. At a meeting of the International Astronomical Union in 2006, astronomers decided to redefine their classification of a planet. And these new rules excluded Eris. Instead of becoming the tenth planet, Eris became a dwarf planet; the same fate as Pluto.

We’ve written many articles about Eris for Universe Today. Here’s an article about how Eris is changing, and here’s an article about how Xena was renamed to Eris.

If you’d like more info on Eris, check out NASA’s page on Eris.

We’ve also recorded an episode of Astronomy Cast that explains why Pluto isn’t a planet any more. Listen here, Episode 1: Pluto’s Planetary Identity Crisis.